Members of the Tumor Necrosis Factor (TNF) family of ligands, so named for their structural similarity to TNF-α, are key components in diverse processes, such as inflammatory responses, cellular immunity and apoptosis. TNF ligands may act locally as type II membrane-bound proteins through direct cell-to-cell contact or as secreted proteins having autocrine, paracrine or endocrine functions. TNF family members bind TNF receptor (TNF-R) family members via their C-terminal extracellular domain. Various TNF family members include TNF, lymphotoxins (LT), Fas, CD27, CD30, CD40, 4-1BB, OX-40, TRAMP, CAR-1, TRAIL, GITR, HVEM, osteoprotegrin, NGF, TRAIN, Kay (BAFF), APRIL and TWEAK (TNF relatedness and weak ability to induce cell death).
A defining feature of this family of cytokine receptors is found in the cysteine rich extracellular domain, initially revealed by the molecular cloning of two distinct TNF receptors. This family of genes encodes glycoproteins characteristic of Type I transmembrane proteins having an extracellular ligand binding domain, a single membrane spanning region and a cytoplasmic region involved in activating cellular functions. The cysteine-rich ligand binding region exhibits a tightly knit disulfide linked core domain, which, depending upon the particular family member, is repeated multiple times. Most receptors have four domains, although there may be as few as one, or as many as six.
TNF family members play a role in the regulation of the immune system, controlling cell survival and differentiation, as well as acute host defense systems, such as inflammation. Continued efforts in the art to manipulate members of the TNF family for therapeutic benefit may provide unique means to control disease. For instance, some of the ligands of this family can directly induce the apoptotic death of many transformed cells, e.g., LT, TNF, Fas ligand and TRAIL. Fas and possibly TNF and CD30 receptor activation can induce cell death in nontransformed lymphocytes which may display an immunoregulatory function.
The ability to induce programmed cell death is an important and well-studied feature of several members of the TNF family. Fas mediated apoptosis appears to play a role in the regulation of autoreactive lymphocytes in the periphery and possibly the thymus. Also, the TNF and CD30 systems have been implicated in the survival of T cells and large cell anaplastic lymphoma lines. Death in this cell line in response to TNF, Fas or LT-β receptor signaling has features of apoptosis.
The TNF family of ligands may be categorized into three groups based on their ability to induce cell death. First, TNF, Fas ligand and TRAIL can efficiently induce cell death in many lines and their receptors most likely have good canonical death domains. Presumably the ligand to DR-3 (TRAMP/WSL-1) would also fall into this category. Next there are those ligands, such as TWEAK, CD30 ligand, and LTalb2, which trigger a weaker death signal limited to a few cells. Studies in these systems have suggested that a separate weaker death signaling mechanism exists. Lastly, there those members that cannot efficiently deliver a death signal. Probably all groups may exert antiproliferative effects on some cell types consequent to inducing cell differentiation, e.g., CD40.
In general, death is triggered following the aggregation of death domains which reside on the cytoplasmic side of the TNF receptors. The death domain orchestrates the assembly of various signal transduction components which lead to activation of the caspase cascade. Some receptors lack canonical death domains, e.g. LTb receptor and CD30, yet can induce cell death, albeit more weakly. Conversely, signaling through other pathways such as CD40 is required to maintain cell survival. There remains a need to further identify and characterize the functions of the TNF family members, thereby facilitating the development of new therapies for TNF family-related diseases.
TWEAK was isolated in a screen for RNA that hybridized to an erythropoietin probe. Chicheportiche et al., J. Biol. Chem. 272:32401-32410 (1997). The mouse and human peptides have an unusually high degree of conservation, including 93% amino acid identity in the receptor binding domain. TWEAK, shown to be efficiently secreted from cells, is abundantly expressed in many tissues, including heart, brain, placenta, lung, liver, skeletal muscle, kidney, pancreas, spleen, lymph nodes, thymus, appendix, and peripheral blood lymphocytes.
One known TWEAK receptor is Fn14, a growth factor-regulated immediate-early response gene that decreases cellular adhesion to the extracellular matrix and reduces serum-stimulated growth and migration (Meighan-Mantha et al., J. Biol. Chem. 274:33166-33176 (1999)). Fn14 has been shown to be induced by FGF, calf serum and phorbol ester treatment and is expressed at relatively high levels in heart, kidney, lung, skin, skeletal muscle, ovary and pancreas tissues, as well as in hepatocellular carcinoma modules and other cancer cell lines, and at lower levels in normal liver tissues.
TWEAK has been implicated in many biological processes. For instance, HT29 cells treated with IFN-γ and TWEAK were shown to undergo apoptosis; although TWEAK's ability to induce apoptosis is weak and only a small number of cell types are susceptible. Chicheportiche et al., J. Biol. Chem. 272:32401-32410 (1997). In contrast, TWEAK has also been shown to induce angiogenesis and proliferation of endothelial cells in a VEGF-independent pathway. Lynch et al., J. Biol. Chem. 274:8455-8459 (1999). Astrocytes are specifically bound and stimulated by TWEAK. TWEAK can infiltrate an inflamed brain to influence astrocyte behavior. Astrocytes exposed to TWEAK secrete high levels of IL-6 and IL-8, as well as upregulate ICAM-1 expression. Saas et al., GLIA 32:102-107 (2000).
TWEAK has also been implicated in immune system regulation. Upon stimulation with IFN-γ, monocytes rapidly express TWEAK, and anti-TWEAK antibodies partially inhibited their cytotoxic activity against human squamous carcinoma cells. A combination of anti-TWEAK and anti-TRAIL antibodies almost completely inhibited cytotoxicity. Nakayama et al., J. Exp. Med. 192:1373-1379 (2000). In contrast, TWEAK mRNA rapidly disappeared in mice treated with lipopolysaccharide (LPS), an inducer of the immuno-inflammatory responses. Furthermore, TWEAK mRNA was also reduced in autoimmune hemolytic anemia and systemic lupus erythematosus in mouse models. These data suggest that the down-regulation of TWEAK expression is an important event in acute and chronic inflammation. Chicheportiche et al., Biochem. Biophys. Res. Comm. 279:162-165 (2000).
Currently, the art lacks a complete understanding of what conditions or diseases are related to TWEAK expression and function, including the role of TWEAK in both inflammatory and non-inflammatory conditions.